(1) Field of the Invention
The invention relates to a method of forming a silicon nitride material cap upon a spin-on-glass silicon oxide layer, and more particularly, to a method of forming a spin-on-glass intermetal dielectric layer with nitridation to prevent poisoned via metallurgy in the fabrication of integrated circuits.
(2) Description of the Prior Art
The spin-on-glass materials have been used for planarization of integrated circuits. The material to be applied is thoroughly mixed in a suitable solvent. The spin-on-glass material suspended in the vehicle or solvent is deposited onto the semiconductor wafer surface and uniformly spread thereover by the action of spinning the wafer. The material fills the indentations in the integrated circuit wafer surface, that is planarization. Most of the vehicle or solvent is driven off by a low temperature baking step often followed by vacuum degassing. Other coatings of the spin-on-glass material are applied, baked and vacuum degassed until the desired spin-on-glass layer is formed.
The final step in the making of the spin-on-glass layer is curing. Curing is a high temperature heating step to cause the breakdown of the silicate or siloxane material to a silicon dioxide like cross linked material. A cured spin-on-glass process is described in U.S. Pat. No. 5,003,062 to Yen.
In the conventional etchback process as taught by Yen U.S. Pat. No. 5,003,062, the spin-on-glass layer is etched back resulting in a poisoned via metallurgy, such as aluminum, caused by outgassing. When a via opening is made through the spin-on-glass, leaving a portion of the spin-on-glass exposed, there may be outgassing of water from the spin-on-glass layer. This water reacts with the aluminum causing corrosion of the aluminum. When etchback is not used, poisoned via metallurgy occurs especially in small via sizes.
The use of a silicon nitride cap on spin-on-glass has been suggested to avoid various problems. "Hot-Carrier Aging of the MOS Transistor in the Presence of Spin-On Glass as the Interlevel Dielectric" by Lifshitz et al, IEEE Electron Device Letters, Vol. 12, No. 3, March 1991, pp.140-142, suggests the use of a silicon nitride cap on the cured spin-on-glass to overcome aging of the MOS transistor. "Field Inversion Leakage in CMOS Double Metal Circuits Due to Carbon Based SOGs" by Pramanik et al, 1989 IEEE VMIC Conference, Jun. 12-13, 1989 pp.454-458, suggests the use of an oxide or nitride film that does not liberate hydrogen gas leading to failure of devices. JA Application 1-192137 dated August 1989 to Shigeki Kimura describes the deposition of an insulating film on the exposed regions of an intermetal cured spin-on-glass layer within a via.
Nitridation of silicon oxide layers is known as shown by U.S. Pat. No. 4,980,307 to T. Ito et al. However, Ito et al requires the use of high temperature in the order of 800.degree. to 1300.degree. C. to accomplish his nitridation of the silicon oxide layer to form a silicon nitride surface layer.